The present invention relates to a method of successfully producing an ohmic contact to a p-type SiC substrate by formation of nickel silicide and titanium carbide.
Most semiconductor devices need terminal connections to carry electric current to and from the internal of the semiconductor device. Such terminal connections, usually called xe2x80x9cohmic contactsxe2x80x9d, must not however impair the semiconductor device itself. Thus, the voltage drop over the ohmic contact should be negligible compared to the voltage drop across other areas of the semiconductor device at the current density in question.
The formation of low specific contact resistance ohmic contacts to p-type SiC is an open issue in SiC-related technology because of the large band gap and electron affinity of SiC. Although a number of metals and metal-like compositions have been tested as contacts to p-doped SiC, no metals with a work-function of about 6 eV are known to form the metal/p-type SiC junction without potential barrier. For this reason, the fabrication of ohmic contacts to p-type SiC even using metals with high work function (such as osmium (Parsons et al., xe2x80x9cOs rectifying Schottky and ohmic junction and W/WC/TiC ohmic contacts on SiCxe2x80x9d, U.S. Pat. No. 5,929,523, (1999)) or platinum (Glass et al., Method of forming platinum ohmic contact to p-type silicon carbide, U.S. Pat. No. 5,409,859, (1995))) still requires a heavily doped (more than 1019 cmxe2x88x923) undercontact layer of semiconductor in order to allow carrier tunneling transport across the interface between the metal and the semiconductor.
Aluminum has been considered as a potential contact metal because it is a doping impurity of p-type in silicon carbide, but its low melting point, 660xc2x0 C., makes it less convenient at high power or high temperature operation. Another problem with aluminum is its reactivity with oxygen that may result in insulating oxides. Thus, the most popular ohmic contacts to p-type 4Hxe2x80x94SiC material are the Al/Ti based metallizations (Furukawa et al. xe2x80x9cSilicon carbide semiconductor device with ohmic electrode consisting of alloyxe2x80x9d, U.S. Pat. No. 5,124,779). Indeed, the Alxe2x80x94Ti alloy is preferred over pure Al due to its higher melting point (1100xc2x0 C. for Al/Ti 90/10 wt. %xe2x80x94ASM Handbook, Vol. 3, xe2x80x9cAlloy Phase Diagramsxe2x80x9d, Ed. H. Baker, p. 2.54) thus, resulting in more thermally stable ohmic contacts. But titanium is characterized by a strong oxygen gettering effect, which causes small amounts of oxygen residues to be captured. These residues later oxidize the aluminum during the contact heat treatment. To overcome this problem, it was proposed to deposit aluminum, titanium and silicon layers (Kronlund et al, xe2x80x9cMethod of producing an ohmic contact and a semiconductor device provided with such ohmic contactxe2x80x9d U.S. Pat. No. 5,877,077) that form an aluminum-titanium-silicide after a heat treatment. During the tri-metal aluminum-titanium-silicide formation any bound oxygen is rejected from the SiC-metal interface but a necessary condition for obtaining a low contact resistance value is that the interface has to move inside SiC during silicide formation. Moreover, in the case of ohmic contacts to p-type SiC with Al-containing silicides or alloys, an additional problem to the above mentioned of contact oxidation, is the aluminum evaporation from the top surface of the deposited metals (J. Crofton, L. Beyer, J. R. Williams, E. D. Luckowski, S. E. Mohney and J. M. Delucca, Sol. St. Electron., Vol 41 (1997), p. 1725). The value of the specific contact resistance is directly related to the quantity of aluminum, which does remain in the metallization. The above analysis clearly shows that, obtaining ohmic contacts with low values of specific contact resistance is a very delicate task for Al/Ti alloys and silicides posing serious problems of reproducibility.
Silicides and carbides are the ideal contact compounds concerning stability at high temperatures. Formation of silicides is more preferable comparing with carbides, because of their lower resistivity. Metal silicide may be formed by two ways: (1) by interaction of the deposited metal with deposited silicon; or (2) by interaction of the metal with silicon carbide. The first case was thoroughly investigated as the metal silicide contains very few carbonaceous species like free elemental carbon, carbides and other carbon-containing compounds that are considered as factors increasing the contact resistance. Stable contacts on n-type 6Hxe2x80x94SiC have been obtained (Tischler et al., xe2x80x9cLow resistance, stable ohmic contacts to silicon carbide, and method of making the samexe2x80x9d, U.S. Pat. No. 5,980,265, (1999)) by depositing a sacrificial silicon layer before deposition of various metals and forming metal silicides following annealing. In addition, the sacrificial silicon layer was in most cases doped with a common dopant of silicon carbide. In this way, a thin top layer of SiC was doped during the annealing step thus reducing the contact resistance. However, for this purpose a long annealing of 1 hour at temperatures higher than 1000xc2x0 C. was necessary. Another possible way to overcome the problem of the produced carbonaceous species during reaction with the SiC is to form simultaneously suicides and carbides with the silicon and the carbon, which are released from SiC during high temperature heat treatment (K. V. Vassilevski, K. Zekentes, G. Constantinidis, N. Papanicolaou, I. P. Nikitina, A. I. Babanin, Mat. Sci. For. Vols. 338-342, p. 1017). In a similar way, ohmic contacts on n-type SiC have been obtained (Bartsch et al. xe2x80x9cSemiconductor device having ohmic contact and a method for providing ohmic contact with such a devicexe2x80x9d WO 00/14805). However, SiC decomposition by this way is not easily controlled and this process is usually undesirable for fabrication of ultrahigh frequency devices usually having shallow p-n junctions. At least, the depth of the SiC decomposition should be well defined and uniform over the sample area.
The present invention concerns a method of producing an ohmic contact to p-type silicon carbide comprising of two layers the first one comprising nickel silicide and the second one comprising titanium carbide. The layers of titanium and nickel are deposited on p-type SiC and an aluminum layer is deposited preferably at the interface between the silicon carbide and the above metals of titanium and nickel. The deposited layers are annealed to convert at least a part of deposited metals to nickel silicide and titanium carbide while aluminum is surface evaporated. The contact is formed by reaction between the metals and the semiconductor, and thus the in-situ simultaneous formation of metal silicide and carbide suppress the release of excess carbon at the contact interface. The presence of aluminum is necessarily employed prior to heat treatment to lower the contact resistivity. Noble metals may be deposited preferably in between titanium and nickel to improve the contact morphology.
In detail, the invention provides a method of forming an ohmic contact to a p-type SiC substrate, comprising the steps of:
(A) depositing the layers of aluminum, titanium and nickel on the p-type SiC substrate; and then,
(B) heat treating the resulting article for sufficient time and at sufficient temperature to convert at least part of it to titanium carbide and nickel silicide by reaction of SiC with the contact metals of titanium and nickel.
The invention particularly concerns the embodiments of such method wherein the heat treatment is carried out at a temperature in the range of from about 850xc2x0 C. to about 1150xc2x0 C.
The invention further concerns the embodiments of the above methods wherein one or more layers of one or more noble metals (especially platinum, gold or palladium species) are deposited preferably in between titanium and nickel contact metals.
The invention further concerns the embodiments of the above methods wherein the depositing layer(s) comprise(s) are formed through a method selected from the group consisting of chemical vapor deposition, sputtering and evaporation from a metal source.
The invention particularly concerns the embodiments of the above methods wherein the p-type SiC substrate is a part of multilayer epitaxial structure or single crystal of SiC. The invention particularly concerns the embodiments of such method wherein the p-type SiC substrate has an acceptor concentration of equal to or greater than about 1018 cmxe2x88x923, and/or the contact has a specific contact resistance to SiC of less than about 10xe2x88x924 ohmxc2x7cm2.
The invention further provides an ohmic contact to a p-type SiC substrate, comprising titanium carbide and nickel silicide, optionally containing one or more layers of one or more noble metals (especially wherein the noble metals are platinum, gold or palladium species). The invention also provides such ohmic contacts, wherein the p-type SiC substrate is a part of multilayer epitaxial structure or single crystal of SiC. The invention also provides such ohmic contacts, wherein the p-type SiC substrate has an acceptor concentration 1018 cmxe2x88x923 and higher. The invention also provides such ohmic contacts, wherein the contact has a specific contact resistance to SiC of less than 10xe2x88x924 ohmxc2x7cm2.
The invention also provides an ohmic contact to a p-type SiC substrate, comprising titanium carbide and nickel silicide, optionally containing one or more layers of one or more noble metals, wherein the contact is made by the method, comprising the steps of:
(A) depositing one or more layers of aluminum, titanium and nickel on the p-type SiC substrate; and then,
(B) heat treating the resulting article for sufficient time and at sufficient temperature to convert at least part of the deposited material to titanium carbide and nickel silicide by reaction of SiC with the contact metals of titanium and nickel.
The invention concerns such ohmic contacts, wherein the heat treatment is carried out at a temperature in the range of from about 850xc2x0 C. to about 1150xc2x0 C., and/or wherein one or more layers of noble metals (especially platinum, gold or palladium species) are deposited preferably in between said titanium and nickel contact metals. The invention particularly concerns such ohmic contacts wherein at least one of said depositing layers is formed using a method selected from the group consisting of chemical vapor deposition, sputtering and evaporation from a metal source.